Watching the Man in the Moon

After the Sun (and the clouds), the Moon is the most obvious big bright thing in the sky. During the full phase, it dominates the night sky, washing out nearby stars and deep sky objects. The monthly cycle has proven a spiritual and intellectual draw to people all over the world and all through history. It showed us the world was round in ancient times, it acted as the first off planet landing site and may even be the first staging post in our first manned interplanetary venture. But what is there to spend time looking at? Ever since Harriot and Galileo sketched the Lunar surface in 1609, telescopes have been trained and eyes brought to bear on this closest of celestial objects. To celebrate this, I mentioned earlier that the SPA in conjunction with the International Year of Astronomy have organised Moon Watches observing the new to first quarter phases.

The Phases of the Moon – naked eye observations

The Moon travels a path through the celestial sphere much as the Sun does. While the Sun moves round the ecliptic line during the course of a year, tracing a path through the signs of the zodiac, the Moon moves a monthly path along another line inclined at up to 5 degrees to that of the Sun. When the Moon is opposite the Sun in the sky, its entire illuminated face shines towards us, it is a full Moon. When it is in the same part of the sky as the Sun (between us and the Sun), it is a new Moon. When it is halfway between these two points, half the face is illuminated and so it is a quarter Moon. Closer to the Sun in the sky than this and a crescent Moon can be seen, further from the Sun than this and a gibbous Moon can be seen. When the Moon is heading from new to full phase and the illumination of the Earthward side increases each night, it is said to be Waxing. When it is heading from a full to a new Moon, getting thinner and thinner each day, it is said to be waning. Hence the phases:

New Moon

Waxing Crescent

Waxing or First Quarter

Waxing Gibbous

Full Moon

Waning Gibbous

Waning or Third Quarter

Waning Crescent

New Moon

The Moon moves eastwards against the background stars at a rate of 12 degrees per day (approximately the distance a clenched fist makes in the sky when held out as far as it’ll go) in order to go through all these phases. This can also be expressed in terms of the Moon moving the equivalent of one full Moon diameter every hour (which is half a degree or 30 seconds of arc).

There is lots to be seen of the Moon with the naked eye. Asides from the same sort of optical phenomena associated with the Sun (Halos with 22 degree diameters, Moon dogs etc) the Moon brings a few of its own. During and close to the time of a new Moon, when the Earth sits behind it and the Sun in front, an observer on the Moon looking back at the Earth would see a ‘full Earth’ (as we are in the position relative to the Moon that the Moon is relative to us during a full Moon). The Earth shines reflected sunlight onto the Moon in the same way as it shines reflected sunlight onto us, which means this Earthshine can illuminate the surface of the new Moon, turning it into a kind of faint full Moon. During this same period of time (and to some extent any time except during a full Moon), the Sun and the Moon can be in the sky at the same time (below is a daytime crescent Moon).

Crescent Moon in the Daytime

The Moon Illusion is another thing that can be seen easily with the naked eye. During the time of a full Moon, catching it rising can sometimes be a startling thing. Normally when looking at the Moon in the sky, there is nothing to compare its size with. When rising close to the ground, a comparison can be made and at full Moon when the Moon rises at the same time the Sun sets, the comparison can be startling. I remember walking down New Oxford Street in London on a summer’s night, I was stopped by a young pedestrian who was turning round everyone walking the opposite way to him so they didn’t miss the effect of the full Moon rising in the middle of the street. If you do see this phenomenon, one way to convince yourself that our trusty satellite isn’t about to slam into us is to curve your fingers to make a tube and put the moon in the middle of the tube, carefully looking with one eye through it. Then close that eye and open your other eye to prove the effect can still be seen outside the tube.

When the Moon is full or new and the inclination is just right (the Lunar plane is inclined to the Solar plane, but as the Sun moves round the sky, it illuminates the Moon from a different area of the sky) an eclipse can happen. At the time of the new Moon, this can be a solar eclipse, where the Moon covers the Sun. During a full Moon, this can be a lunar eclipse, when the shadow of the Earth passes over the Moon. Below is a picture of the Sun projected onto a screen during a partial eclipse over London. Below that is a picture of the Eclipsed Moon over Newton Street in London.

The Moon partially eclipses the Sun

The shadow of the Earth

In the solar eclipse photograph, one thing that can be seen are ridges on the edge of the Moon – valleys and mountains in silhouette. The eclipsed Moon is similarly full of interesting detail, the red band falling across the Moon is the shadow of the Earth’s atmosphere. Red light has been bent round to shine onto the Moon. This band and the dark shadow of the Earth that comes with it at fuller eclipses convinced the ancients that the world was round.

With the naked eye, there are a few features of the Moon that can easily be seen – the darker Maria and the lighter Terrae for instance. The Maria are believed to be ancient impact craters flooded with magma. The Terrae are the highlands, covered with more reflective lunar regolith, or soil, which although more reflective than the stuff in the Maria is actually only as relfective as coal, returning about seven percent of the light it gets. These light and dark patches are sometimes thought of as forming the Man in the Moon – visible in the lunar eclipse picture above. 59% of the lunar surface faces the Earth due to lunar libration, where the Moon seems to roll around a bit in the sky, though it takes a few photographs to notice it.

Viewing the Moon through telescopes or binoculars

The Moon is an extremely bright object and looking at it through a telescope or binoculars can be painful in the full phase. Below is a picture of fellow Demonstrator Daniel Went looking through the Radcliffe telescope at ULO. The telescope is a massive thing and is pointed directly towards the Moon. However, due to the fact that the telescope only looks at a small portion of the Moon’s full disc, only a small portion of all the light from the Moon gets through to the eye (the density of the brightness, or luminosity, decreases with increasing magnification, once the whole Moon has filled the eyepiece). It was still pretty painful though… Lunar filters are available as are ‘stops’. Some telescopes have holes in their lens caps, which themselves have caps. The idea is to block out some of the incoming light so objects at the eyepiece are fainter. Other telescopes have lunar filters, much as you can buy solar filters.

Dan looks at the Moon through the Radcliffe

But what can be seen through the eyepiece? Well, craters perhaps. Yes, lots of them, but there are different types of craters – some craters have terraced walls, some have central peaks or dips, some are surrounded by smaller craterlets and some have been formed on top of others. Some craters are surrounded by light ejecta, the residue from the impact still visible. There are also features that aren’t craters – valleys, mountain ranges, a huge wall formed by a previous geological fault, rilles, which are deep thin valleys that can extend for many kilometres and wrinkles in the Maria. Below are some images of a crater field. This is not only to show what can be seen in a crater field, but also to show how magnifying an object reduces its luminosity. They are both afocal shots through the 10 inch Fry telescope, but the first uses no camera zoom, the second uses three times optical zoom in addition to the eyepiece used.

Moon through the Fry

Magnified Moon

You may notice that the Moon has altered a little between the shots. That is because the Fry telescope follows the rotation of the stars, whereas the Moon moves against the stars, which is visible at this magnification. Other times that the motion of the Moon may be visible include during conjunctions and occultations. A conjunction is just when different objects (Like the Moon and Venus on a few occasions recently) appear in the same area of the sky. A lunar occultation occurs when the Moon passes in front of something (a total solar eclipse is technically a lunar occultation). The Moon can be seen to visibly move over the star or object and hide it from view. In an hour or so, that object pops out the other side. It should be noted that the sides of the Moon aren’t actually smooth and round, but craggy from all the mountains and craters. A close look at the blown up version of the projected eclipse picture above does show this cragginess.

Photographing the Moon

Now you’ve seen it, you might want to snap it. Taking photographs of the Moon is a little trial and error. The Moon can be snapped using just a camera and its lens, a camera up against a telescope eyepiece or a camera mounted onto a telescope. Different f-ratios and films will produce different brightnesses. A fast film (one with a higher ISO rating) will produce a brighter image faster than a slow film, but it will be granier. A fast f ratio will do the same. The focal ratio is the focal length of the lens or mirror divided by the aperture diameter, faster ones are lower ratios – an f/5 is faster than an f/16. Fast focal ratios are preferred for faint objects such as deep sky objects rather than planets and the Moon as they can wash out detail. If you have a camera with a lens and a telescope with an eyepiece, the focal ratios of the two can be multiplied to give the effective focal ratio. Below is an idea of the exposure times required to photograph the Moon with a film of ISO 400 and f/16

Full Moon – 1/250 seconds

Gibbous Moon – 1/125 seconds

Quarter Moon – 1/60 seconds

Thick Crescent Moon – 1/30 seconds

Thin Crescent Moon – 1/15 seconds

Earthshine – 20 to 40 seconds

For higher or lower ISOs, the times are inversely proportional – ie, if your ISO speed = 800, this is 2 x 400, so DIVIDE exposure times by 2. You might also be interested to know that if you divide the focal length of a lens by 109, you will obtain the diameter of the Moon in millimeters on film. As I mentioned earlier, the Moon moves against the background stars and this can create a problem. If you are taking a long exposure, perhaps in order to use a cleaner slower film, you might find everything is blurred by motion. Even if your tripod is moving at a sidereal rate (the same as the speed the stars move), the Moon still moves against the stars and will still blur after a while. If you have a 50mm lens, you’ll have 12 seconds to get a shot on a fixed tripod and 4 minutes to get the same shot on a motor driven tripod moving at the same rate as the stars. If you have a larger lens, then the time you have to get a shot is inversely proportional to lens size.

Below is a shot of the same crater field seen in the Fry photographs above, but this time taken with the larger Radcliffe telescope. An interesting project, first done by Galileo, is to work out the sizes of lunar features from the lengths of their shadows. You need to know the angle the Sun shines down from as well as the angle the shadow is at to you (to combat shortening of the shadows through tricks of perspective). Craters at different points of the lunar disc have different length shadows due to the curvature of the Moon (the Sun makes a different angle, 90 degrees for the sun facing spot, 0 degrees at the terminator). It can be quite challenging, but also quite rewarding. Galileo himself wasn’t too bad at this.

Shadows on the Moon

Finally, here’s a shot of the entire quarter Moon taken through a seven inch Meade Maksutov telescope, with f/15 and 1/15 exposure time. It was very cloudy, there was some rain, but I managed to get a few photographs including this, one of my first astrophotographs in 1999 at Mill Hill observatory.